1. Field of the Invention
The present invention relates to a porous ceramic product and to a method for its manufacture, which product can be employed in particular as lightweight brick.
2. Brief Description of the Background of the Invention Including Prior Art
Ceramic materials are in general provided with a porosity which depends on and is generated by the composition of the raw materials, the grain structure of the raw materials and the driving out of the water during drying and firing. This "natural" porosity is insufficient for application situations where it is important to assure a good thermal insulation in addition to the refractory properties. Ceramic products are provided with additional pores for this purpose. This incorporation of pores can be generated with so-called "opening materials". These materials are additives which burn, fuse off, melt out, evaporate or gasify and are provided as particles or the like which are mixed into the ceramic mass and which take up a predetermined volume. In the case of a temperature increase these materials escape as a vapor, as a gas or as a liquid sometimes without leaving residues, where the space originally taken by the particles remains as a pore volume, whereby the pore incorporation results, that is the additional porosity besides the "natural" porosity.
The "opening materials" are in most cases organic materials such as saw dust, cork powder, porous polymerized styrenes, naphthalene or the like. In addition inorganic, porous additives are known such as perlite, foamed glass, vermiculite, expanded clay granulates, diatomaceous earth, kieselguhr or the like, which can lose their own porosity during the ceramic firing precess by way of melting down, where possibly a reaction with the matrix material occurs and a hollow space or, respectively, a pore is formed at the place where the grain of the additive material was disposed before. The use of foaming and/or swelling chemicals, which generate a ball shaped porosity before and during the firing of the ceramic material provides another way for generating pores in ceramic products.
The majority of the additives are more or less of a ball shaped grain structure. Saw dust and fiber materials are an exception in this regard. Since the specific weight of the additives is in general relatively low compared to the other ceramic raw materials to which they are admixed, it is in general necessary to make special provisions so that an effective mixing is assured, that is, the dispersion of the additives in the ceramic raw mass is achieved, and this in turn results in a homogeneous pore distribution of the pores in the fired product. In most cases the homogeneous distribution is not achieved because the additives are provided with a wide grain size distribution and the small particles behave differently from the large particles during the mixing process. Even in the case where an additive with a narrow grain size distribution is employed it is hardly possible to realize homogeneous distributions. The quality of the fired products suffers from the inhomogeneous distribution, not only with regard to the strength, but also, in particular, with regard to its thermal conductivity properties.